1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Various measuring systems and apparatuses have been hitherto suggested in order to know the concentration of a predetermined gas component in a measurement gas.
For example, those known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized while using a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia to measure an electromotive force generated between the both electrodes.
The sensor as described above suffers the following problem. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in a combustion gas as a measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise. Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform measurement for a combustion gas produced under such a combustion condition.
A system has been disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 63-38154 and 64-39545, in which a pair of electrochemical pumping cell and sensor cell comprising Pt electrode and an oxygen ion-conductive solid electrolyte are combined with another pair of electrochemical pumping cell and sensor cell comprising Rh electrode and an oxygen ion-conductive solid electrolyte to measure NOx in accordance with a difference between respective pumping current values.
Further, for example, Japanese Laid-Open Patent Publication Nos. 1-277751 and 2-1543 disclose the following method. That is, two pairs of electrochemical pumping cells and sensor cells are prepared. The limiting pumping current is measured at a partial pressure of oxygen at which NOx is not reduced, by using a sensor comprising one of the pairs of pumping cells and sensor cells, while the limiting pumping current is measured at a partial pressure of oxygen at which NOx is reduced, by using a sensor comprising the other pair of pumping cell and sensor cell so that the difference between the limiting pumping currents is determined. Besides, the difference in limiting current is measured by using a sensor comprising a pair of pumping cell and sensor cell, while switching the partial pressure of oxygen in a measurement gas between a partial pressure of oxygen at which NOx is reduced and a partial pressure of oxygen at which NOx is not reduced.
In principle, the output value obtained from the gas sensor as described above involves strong dependency on temperature, and hence it is necessary to perform temperature compensation. The temperature of the gas sensor is correlated with the alternating current resistance (impedance) of the gas sensor. Specifically, the impedance of the gas sensor is lowered as the temperature of the gas sensor is raised.
A constant resistance control method based on the use of a bridge has been hitherto employed as a technique for performing temperature compensation for the gas sensor. According to the constant resistance control method, the total resistance of a heater (=resistance of heat-generating section of heater+resistance of heater lead section) is controlled on the basis of the temperature of a measurement gas.
As described above, the total resistance of the heater is controlled in the constant resistance control method. Accordingly, when the resistance value of the heater lead section contained in the element is increased in accordance with the increase in measurement gas temperature, the control is made so that the resistance value of the heat-generating section of the heater is decreased. As a result, a phenomenon occurs in which the output of the heater is lowered.
Such a system involves the following inconvenience. That is, the temperature at a portion for sensing the predetermined gas component may be deviated from a predetermined designed value. As a result, a phenomenon occurs in the output characteristic of the gas sensor, in which the detection current value is shifted with respect to the concentration of the predetermined gas component. Specifically, the detection current value is increased, as the temperature is raised, as compared with a prescribed detection current value which is expected based on the concentration of the predetermined gas component, and thus the detection accuracy is deteriorated.
In order to dissolve the foregoing inconvenience, it is necessary to decrease the resistance value of the heater lead section as small as possible, resulting in a problem that the degree of freedom is lowered concerning wiring arrangement design.
A gas sensor has been hitherto suggested, as an alternative of the constant resistance control method described above, which comprises a means for measuring an impedance of the gas sensor, and a current control unit for controlling electric power application to a heater so that the impedance of the gas sensor is constant (see, for example, Japanese Laid-Open Patent Publication No. 58-178248). In the gas sensor described above, an amount of alternating current is superimposed on an electric power source, while a circuit is provided for detecting the impedance generated in accordance therewith. Thus, the current to be supplied to the heater is controlled so that the impedance is constant.
Further, a gas sensor has been also hitherto suggested, in which only a direct current component is positively feedback-controlled in order to avoid oscillation in a current control system (feedback control system) for a heater on the basis of a detected impedance (see, for example, Japanese Utility Model Publication No. 7-45004).
However, in the case of the gas sensor including the conventional heater current control system, the alternating current flows to the detecting electrode which is used to detect the predetermined gas component. For this reason, the alternating current appears as a noise, and it is feared that the S/N ratio of the detection output is deteriorated.